Flat-panel display

ABSTRACT

A flat-panel display comprises a projector ( 21-23 ) illuminated by a collimate light source such as an infra-red laser, a waveguide ( 1 ) which ejects rays at a distance along the axis of propagation related to their angle of injection into the edge of the waveguide, an input slab ( 3 ) for magnifying the projected image in the width dimension in the plane of the panel perpendicular to the axis of propagation, and a phosphor output screen which converts the infra-red light to the visible. The use of monochromatic light within the waveguide reduces dispersion problems, and the use of a triad of projectors, each injecting at a different angle, makes registration relatively simple.

This invention relates to a way of making a flat-panel display by usinga laser diode to project the image from a microdisplay into a taperedpanel.

Flat-panel displays which have screens large enough to stimulate thequick reactions of our peripheral vision will give pictures greatimmediacy, yet because they are flat the displays will fit easily ontothe wall of a room. The size of conventional flat-panel displays howeveris limited by the resistor/capacitor time constant of the row and columntransparent conductors, and by the area over which lithography can besufficiently precise to make transistors. The cost of makingactive-matrix liquid-crystal displays with screen diagonals greater thanone metre is prohibitive, and even the cheaper plasma displays are tooexpensive for most uses, but costs reduce considerably with size and the2″ by 2″ (5 cm×5 cm) liquid-crystal displays used in video projectorsare relatively cheap, while fingernail-sized microdisplays look set tocost only a few dollars.

Video projectors comprise a two-dimensional display, a projection lensand a translucent screen, the projection lens forming on the translucentscreen a magnified image of the two-dimensional display which can bealmost as big as one wants. Video projectors are cheap, so are becomingincreasingly popular, but if the projector is pointed at the front ofthe translucent screen then often the projector gets in the way of theviewer, or the viewer gets in the way of the projected light.Furthermore, unless the room lights are dimmed, the image looks washedout because the screen scatters background light as well as theprojected image. The projector can instead be pointed at the rear of thescreen so that there is nothing between the viewer and the screen, andthe screen made to scatter only light incident on its rear, butrear-projection displays are bulky.

Recently there was disclosed in PCT/GB01/01209 a tapered display whichcomprises a video projector and a sheet of glass tapering in onedirection. The video projector is itself composed of a source ofapproximately collimated illumination, a microdisplay, a condensing lensand a projection lens, and as the rays leave the projection lens theyform a narrow waist. At this point the rays are passed into the thickend of the tapered sheet of glass. When a ray is shone into the thickend of a sheet of glass which tapers, then the out-of-plane anglemeasured with respect to one face of the taper will change each time theray reflects off the opposite face of the taper. Eventually, as the raypropagates far enough along the taper, the out-of-plane angle becomesgreater than the critical angle, and at this point light escapes thetaper. The distance into the tapered sheet of glass at which the rayleaves the taper is therefore determined by the angle at which the rayis injected. In this way the 2D array of pixels on the microdisplay ismapped one-to-one to a 2D array of pixels on the face of the taperedwaveguide. An anti-reflection coating is needed to ensure that all thelight leaves the screen when the ray reaches the critical angle, sinceotherwise there is blurring between adjacent rows of the image.

The tapered display shares many of the advantages of a rear-screenprojection display, but works better with laser illumination than withillumination from an incoherent white light source. This is firstbecause white light is not collimated, and secondly because it comprisesmany wavelengths so the anti-reflection coating has to be morecomplicated and is more expensive. Red, green and blue lasers aregetting more powerful and cheaper, but are likely to be more expensivethan arc-lights for several years, and a coating designed even for justthree wavelengths is complicated.

Recently there was disclosed by A Rapaport, F Szipocs, J Milliez, HJenssen, M Bass, K Schafer and K Belfield in “Optically Written DisplaysBased on Up-Conversion of Near Infrared Light”, Conference record of the20th International Display Research Conference of the Society forInformation Display, pages 111 to 114, a way of making a video displayin which the beam from a modulated 975 nm infra-red laser was scannedacross a screen of Yb³⁺ ions which absorbed the light and transferredthe energy to dopant ions which fluoresced at visible wavelengths.Infra-red laser diodes are powerful and relatively cheap, and red, greenand blue emissions were demonstrated using Tm³, Ho³ and Er³ ionsrespectively in a NaYF₄ host with the potential for high opticalefficiency, but the way in which the image is projected is bulky.

According to the present invention there is provided a flat-paneldisplay comprising a projector illuminated by a collimated light sourcesuch as an infra-red laser, a waveguide which ejects rays at a distancealong the axis of propagation, that is related to their angle ofinjection into an edge of the panel, means, preferably between theprojector and the waveguide, for magnifying the projected image in thewidth direction (i.e. the dimension in the plane of the panelperpendicular to the axis of propagation), and an output screen whichconverts the infra-red or other source light to the visible.

Using the invention it is possible to use a single, cheap, high-powerlaser, which is well adapted as an input device for the wedge-shapedwaveguide, and still obtain a full-colour image. The output screen canbe made of phosphor material, in RGB pixels for a colour image, whichhas the advantage of emitting uniformly in all directions, giving aneasily visible image. Infra-red light is advantageous because mostuseful glass and plastics materials are transparent to it. Monochomaticor narrow-band light is advantageous because of the absence ofdispersion and the greater simplicity of coatings such as antireflectioncoatings.

For a better understanding of the invention embodiments will now bedescribed by way of example with reference to the accompanying drawings,in which:

FIG. 1 illustrates how the distance which a ray of light propagatesalong a tapered waveguide is determined by the angle at which the ray isinjected;

FIG. 2 illustrates how a screen of up-converting phosphors and a tapereddisplay illuminated by an infra-red laser diode can give a visiblepicture;

FIG. 3 illustrates how each of three video projectors can be used toilluminate pixels from one set each of three sets of phosphors; and

FIG. 4 is a close-up of the phosphor system of FIG. 3.

FIG. 1 illustrates the principle of light escape from a tapered panel,at a location dependent on the out-of-plane angle of injection. Thisprinciple is used in PCT/GB01/01209 to make a flat-panel 2-D display.

In FIG. 2 the beam from an infra-red laser diode, 10, is expanded by aninverted telescope pair of lenses 11, 12 then passed into a videoprojector comprising a microdisplay 2 a and a condensing lens 2 b. Themodulated rays, reflected by a mirror 6, then enter the edge of awedge-shaped slab of glass or transparent plastic 3 to which they areconfined by total internal reflection at the glass/air interface. Therays propagate within the plane of the slab so that the ray bundle hasexpanded to the width of the slab 3 by the time they reach its end. Therays may then optionally be collimated by a cylindrical lens 4sandwiched on each face of the slab between a pair of front silveredmirrors 5 a, 5 b, and then they enter the thick end of a tapered sheetof glass 1. The front (viewer-side) surface of this tapered sheet ofglass 1 is coated with an anti-reflection coating tuned to thewavelength of the laser diode 10 so that rays leave the tapered sheet 1as soon as they reach the critical angle, and the rays are then incidenton a sheet of up-converting phosphors 7 and produce a magnified image invisible light of the pattern modulated by the microdisplay.

A colour picture is desirable and to this end the phosphor screen 7could be patterned with a mosaic of red, green and blue up-convertingphosphors and the microdisplay registered to these so that one colour oranother could be controllably illuminated. However, this would requireprecise registration of the microdisplay and phosphor screen.Alternatively a shadow mask could be used in the same way as for acathode-ray tube, but this wastes light.

FIG. 3 shows how three microdisplays 21, 22, 23, each illuminated by aninfra-red laser diode (not shown), can control one colour each on thescreen 7 via a set of lenses. Rays from each of the microdisplays 21,22, 23 project up through the slab 6 and are collimated by thecylindrical lens 4 so that the rays from each microdisplay have the samein-plane angle, but one which is different from that for rays from theother microdisplays. Consequently as they emerge from the wedge 1, raysfrom any one microdisplay will have a distinct azimuthal angle. A seriesof prisms 19 bends the rays into the horizontal plane (i.e.perpendicular to the slab), and then a vertical array of cylindricalmicrolenses 14 is used to focus the rays to sets of points in verticalcolumns, and one up-conversion phosphor colour is placed at these setsof points. The same is done in turn for the other microdisplays, so thatmicrodisplay 21 modulates phosphors 18, microdisplay 22 modulatesphosphors 17, and microdisplay 23 modulates phosphors 16.

Phosphors emit light in all directions, and it is desirable that lightemitted to the rear of the screen be reflected back towards the front.However if a simple mirror is placed behind the phosphors, this willalso reflect the infra-red light intended to illuminate them. Onesolution is to use a dichroic mirror which transmits infra-red light andreflects the visible, but dichroic mirrors are expensive. Instead, FIG.3 shows a horizontal array of cylindrical microlenses 13 between theprisms 19 and the vertical lenses 14 which focus the incident infra-redlight through horizontal slits in a mirror 15. Because these slits aresmall, most of the visible light from the phosphors is reflected backtowards the front of the screen.

FIG. 4 shows an expanded view of one RGB colour pixel of the phosphorscreen. The RGB phosphors can be of the type described by Rapaport etal. However, they could also be excited by UV radiation. It should benoted that although ultra-violet lasers are still expensive and weak,mercury discharge lamps emit light with a narrow linewidth and from sucha small aperture that it can be moderately well collimated with aparabolic mirror. Conventional phosphors can be used to convert thisultra-violet light to the visible, and the layouts of FIGS. 2, 3, and 4will work also with ultra-violet light.

1. A flat-panel display comprising a projector (2 a, 12; 21-23)illuminated by a collimated light source such as an infra-red laser(10), a waveguide (1) which ejects rays at a distance along the axis ofpropagation that is related to their angle of injection into an edge ofthe waveguide, means (3) for magnifying the projected image in thedimension in the plane of the panel perpendicular to the axis ofpropagation, and an output screen (7) which converts the collinatedlight to the visible.
 2. A flat-panel display according to claim 1, inwhich the waveguide (1) is a tapered slab of transparent material.
 3. Aflat-panel display according to claim 2, in which the output screen (7)is a phosphor sheet on or near one face of the slab.
 4. A flat-paneldisplay according to claim 3, in which the phosphors are RGB phosphorsin columns parallel to the direction of propagation along the slab, andthe projector has three corresponding colour projectors (21-23)injecting at different in-plane angles.
 5. A flat-panel displayaccording to claim 4, further including means (14) for converging lightfrom each projector on to columns of the corresponding phosphor.
 6. Aflat-panel display according to claim 5, in which the light approachingthe phosphors is converged, e.g. by a set of cylindrical microlenses(13), and made to pass through corresponding slits in the row direction,so that little light emitted backwards from the phosphors passes backinto the waveguide system.
 7. A flat-panel display according to claim 3,in which the or each projector is an IR laser and the phosphors areup-converting phosphors.
 8. A flat-panel display comprising a projector(2 a, 12; 21-23) illuminated by a collimated light source, a waveguide(1) in the form of a panel having an edge, which ejects rays at adistance along the axis of propagation that is related to their angle ofinjection into the said edge of the waveguide, means (3) for magnifyingthe projected image in the dimension in the plane of the panelperpendicular to the axis of propagation, and an output screen (7) whichconverts the light ejected from the waveguide to visible light.
 9. Aflat-panel display according to claim 8, in which the waveguide (1) is atapered slab of transparent material.
 10. A flat-panel display accordingto claim 9, in which the output screen (7) is a phosphor sheet on ornear one face of the slab.
 11. A flat-panel display according to claim10, in which the phosphors are RGB phosphors in columns parallel to thedirection of propagation along the slab, and the projector has threecorresponding colour projectors (21-23) injecting at different in-planeangles.
 12. A flat-panel display according to claim 11, furtherincluding means (14) for converging light from each projector on tocolumns of the corresponding phosphor.
 13. A flat-panel displayaccording to claim 12, in which the light approaching the phosphors isconverged, and made to pass through corresponding slits in the rowdirection, so that little light emitted backwards from the phosphorspasses back into the waveguide system.
 14. A flat-panel displayaccording to claim 10, in which the or each projector comprises an IRlaser and the phosphors are up-converting phosphors.
 15. A flat-paneldisplay according to claim 13, wherein the light approaching thephosphors is converged by a set of cylindrical microlenses.
 16. Aflat-panel display comprising: a projector (2 a, 12; 21-23) including acollimated light source in the form of a laser (10), a panel-shapedejecting waveguide (1) having an edge into which light is injected fromthe laser so that it propagates along an axis, while being confined bytotal internal reflection, and which ejects rays at a distance along theaxis of propagation that is related to their angle of injection into thesaid edge of the waveguide, the ejecting waveguide having ananti-reflection coating tuned to the wavelength of the laser light; aslab waveguide (3), having the same width as the ejecting waveguide, formagnifying the projected image from the projector in the dimension inthe plane of the panel perpendicular to the axis of propagation beforeit is injected into the ejecting waveguide; and an output screen (7) inthe form of a phosphor sheet on or near one face of the panel-shapedwaveguide, which converts the ejected light to the visible.